Here are the official add-ons and patches for Half-Life, and a couple of bots. Note that you don't have to download patch files manually - you can use the Sierra Utilities shortcut installed with Half-Life to automatically update the game to the latest version.

To find out which version of Half-Life is installed, either check the console within the game, or the Version tab of the hl.exe file Properties. For reference, the original retail version was 1.0.0.5, and the 'Game of The Year' special edition was 1.0.1.1.

There's also a patch available for running Half-Life on PCX2-based cards - if that's your bag, check out the PowerVR page.

Full client patch to update Half-Life to 1.1.1.0. This patch will also update any Opposing Force installation to 1.1.0.9, although it only appears to update the engine; see the 1.1.0.8 update below for more info.

Steam is a broadband distributed file system which Valve will be using to distribute it's games. Game updates are automatic, and it also includes an instant-messaging client (which also works in-game) and a server browser. Whether Steam is going to take off isn't clear right now, but the idea looks promising...

Patch to update the English Language version of Opposing Force to 1.1.0.8. This includes multiplayer fixes and a couple of extra maps. Note that the Opposing Force 1.1.0.9 update included in the Half-Life 1.1.1.0 update doesn't include the maps; consequently, you might be best advised to update Opposing Force to 1.1.0.8 before applying the Half-Life 1.1.1.0 update. If you've already applied Half-Life 1.1.1.0 but you don't have the new maps (look for op4cp_park.bsp in the gearbox/maps folder), you can apply the 1.1.0.8 Opposing Force update, then apply the OpForUp.exe update that came with the Half-Life 1.1.1.0 update (you'll find it in your Half-Life folder).If Sierra release a seperate Opposing Force 1.1.0.9 patch, I'll post it here.

The HPB Bot, version 2.1a, which is compatible with (and requires) Half-Life 1.1.0.9 or later. improves TFC support. If you're upgrading the HPB Bot, be sure to remove the previous version first using the remove.bat file supplied. Check the HPB_bot_ReadMe.txt file that comes with the bot for more information.Note that the HPB bot works with the following mods: Half-Life deathmatch (and teamplay), Team Fortress Classic (TF 1.5), Front Line Force, Half-Life Opposing Force, Holy Wars, Counter-Strike (but the bots don't buy weapons) and Deathmatch Classic.

Half-Life Multiplayer Add-on Pack. this includes four new models, a new multiplayer map (Team9) and loads of new decals. Once you've downloaded it, unzip it to the folder above your Half-Life installation.